1. Field of the Invention
The present invention relates to a method for fabricating a dual-gate semiconductor device, and more particularly to a method for fabricating a dual-gate CMOS device, which allows poly depletion effect to be minimized.
2. Description of the Prior Art
As the size of semiconductor devices becomes smaller, three-dimensional poly depletion effect occurs. To minimize this poly depletion effect, poly-Si doping needs to be increased.
As used herein, the term “three-dimensional poly depletion effect means that the depletion caused by vertical electric fields in a gate due to insufficient poly-Si doping, and the gate length-direction depletion resulting from fringing electric fields at the gate corner, and also the gate width-direction depletion caused by an increase in poly thickness of the STI edge when effective fox height is higher than zero, appear in a composite manner.
The poly depletion effect results in an increase in threshold voltage (Vt), a reduction in current and an increase in variation of Vt, and also can act as the main cause of low Vcc soft bit fail in the case of FCMOS SRAM.
This poly depletion effect can be effectively minimized by increasing the dose or energy of N+ ion implantation into poly-Si. However, the increase of N+ ion implantation dose disadvantageously causes the curing of photoresist (PR) used as a mask after ion implantation, so that PR residues remain even after a removal process of PR.
Furthermore, the increase of N+ ion implantation energy is a way of implanting a sufficient amount of impurity ions to the bottom of poly-Si (near a boundary with a gate insulting film) so as to minimize the poly depletion effect. However, excessively high implantation energy causes so-called “penetration” where impurity ions penetrate poly-Si into a channel region under the poly-Si to cause a reduction in Vt and an increase in variation of Vt. Thus, since the increase of ion implantation energy has risk, the amount of a suitable increase in ion implantation energy needs to be selected.